1. Field of the Invention
This invention relates to a diesel particulate filter apparatus adapted to collect by a filter an exhaust gas discharged from a diesel engine, and incinerating the same.
2. Description of the Prior Art
The combustion in a diesel engine is based on the heterogeneous mixing carried out by injecting a fuel into high-temperature and high-pressure air. In the heterogeneous mixing, air and a fuel are not uniformly mixed unlike those in a homogeneous gaseous mixture. Therefore, the carbon component in the fuel turns into soot and HC due to the high combustion heat, and they are agglomerated into particulates, which are discharged to the outside to cause the contamination of the environment. A conventional diesel particulate filter using a porous filter, one of exhaust emission control equipment that treat exhaust gases from engines, has a large area to collect particulates. To incinerate the particulates accumulated on the filter, the particulate filter is provided on its surface with a heater, which, when the amount of accumulated particulates exceeds a predetermined reference, is applied an electric current to incinerate particulates, thereby regenerating the filter.
Japanese Patent Laid-Open No. 256812/1990 discloses a particulate trap which can be electrically regenerated. This particulate trap comprises a gas permeable support, a plurality of fiber layers of ceramic fiber, and a heater, and the fiber layers and heater are provided in a space extending radially from the gas permeable support. An exhaust gas is introduced into clearances between the fiber layers, and the particulates in the exhaust gas are collected in a position near the heater, the collected particulates being incinerated by supplying an electric current to the heater. The material for the ceramic fibers is selected from continuous molten silica, glass, alumina silica, zirconia silica, alumina chromia, titania, graphite, silicon carbide and alumina boria silica. The ceramic fiber is directly braided on or wound around the surface of the gas permeable support or heater.
It is necessary that the diesel particulate filter be made compact so that it can be mounted on a vehicle conveniently and collect particulates, such as carbon, soot and HC efficiently. The particulates easily burn by reacting with oxygen but require high temperatures for combustion, and hence they cannot completely burn only with the exhaust gas heat. A conventional diesel particulate filter apparatus is provided with two filters of the same size arranged in parallel with a flow of an exhaust gas. When one filter is clogged with collected particulates, the exhaust gas is supplied to the other filter, while the air required for the incineration of particulates is sent to the clogged filter so as to heat the filter and incinerate the collected particulates, whereby the filter is regenerated.
When the amount of particulates collected by the filter exceeds a predetermined amount, the heater is turned on for heating. It is, however, very difficult to precisely detect the amount of particulates accumulated on the filter. The conventional method, for example, measures the amount of accumulated particulates indirectly by measuring the exhaust gas pressure with a pressure sensor installed in the exhaust gas flow upstream of the filter. The exhaust gas pressure and flow, however, vary depending on the load and revolution of the engine, making it difficult to detect the precise amount of particulates. The filter's exhaust gas resistance pressure assumes different values for a full load and a part load, as indicated by A and B in FIG. 7. The allowable value of the filter's exhaust gas resistance pressure P when the engine revolution is high at N.sub.2 is different from the resistance value of the exhaust gas resistance pressure P when the revolution is low at N.sub.1. In FIG. 7, the revolution N.sub.0 represents the engine revolution at idling. Even in the state of A where the filter is clogged and no longer suited for service, the filter continues to be used without being regenerated. If, after this, the engine is suddenly applied a high load, the exhaust gas resistance pressure increases sharply exceeding the allowable value, with the result that the filter cannot withstand the high exhaust gas resistance pressure.
The ability to detect, as the absolute value, the amount of particulates accumulated on the filter such as carbon, soot and HC ensures the most reliable control of the regeneration timing for the filter and is also required to improve the durability of the filter. Therefore, it has been the task to establish the way to detect the amount of particulates accumulated on the filter as an absolute value.